Paul Rudd once took off all his clothes and ran around a movie theatre because he "thought it would be funny".

The actor believes people get a different perception of a movie if they view it on their own as opposed to in a packed theatre.

The 43-year old comedian stars in movie This Is 40, and while promoting the film explained he once engaged in some unusual behaviour when he thought he was alone in a cinema.

"This happened to me once and I think you view a movie differently [when you're the only person in the theatre]," Rudd shared with BBC Radio One's Breakfast Show.

"Once I walked in and me and my friend were the only people there, we felt weird for sitting next to each other. Even if there's one other person who comes in, it's not weird, but if it's just you and your friend, you feel a little uncomfortable.

"Once we started running up and down the aisles and doing crazy dancing when we were the only ones in the theatre... then on one occasion I thought it would be funny to take off all my clothes and run around the theatre naked... then I realised there was a guy in the projection room watching me and laughing the whole time."

This Is 40 is a sequel to 2007's film Knocked Up, but this time focusing on Rudd and Leslie Mann's characters Pete and Debbie.

Rudd joked that shooting the Judd Apatow-directed picture wasn't as demanding as making an epic movie like Lincoln, especially as he isn't too far removed from Pete.

"It took two-and-a-half to three months to finish shooting this one. It's not like I'm playing Lincoln, I'm not Daniel Day-Lewis wearing the clothes for 24 hours a day. Even though it's a character it's not an amazing transformation. It's not a struggle to play," Rudd laughed.

"I don't get too bummed out about working every day, it's still pretty cool. I just finished a play a few months ago, now that's tough. Eight shows a week - that's more demanding than a movie. Sometimes I don't like doing that at all, the days you think, 'Oh I have to go in,' and that sucks."